Its name most probably comes from the phrase miejsce żarne - burned-out area, and refers to burning of forests in ancient times, to make fields.

Żarnów has a long history, in the early years of Polish state it was a major urban center of the country.

It was home to one of the oldest Polish castellanies, with a church founded in 1111 by prince Bolesław III Wrymouth as a votum for his victories in a war with Pomeranian tribes.

In 1136, a papal bull of Innocent II mentioned Sarnov as one of major grods of central Poland, together with other administrative centers of the area - Skrzynno and Małogoszcz (the three towns made the so-called Pilica castellanies).

Following the joint German-Soviet invasion of Poland, which started World War II in September 1939, it was occupied by Germany until 1945.